1. Field of the Invention
This invention relates to rotary pumps and more specifically to double rotary piston pumps.
2. Related Art
Conventional double rotary piston pumps operate by rotating a pair of cylindrical, i.e., generally disc-shaped, pistons within cylindrical chambers about offset parallel axes and in opposite directions. This rotation is typically accomplished by applying a driving rotation via a drive shaft or drive gear to the pistons at points offset from their centers to provide cooperating eccentric motion to each piston. To maintain pumping efficiency, the pistons should remain in mutual contact during the entirety of each cycle or period of rotation. However, in pumps of this type in the prior art, the pistons tend to move apart and lose contact with each other ("gap") during a portion of each cycle of rotation. This gapping phenomenon is caused by relative translation movement of the eccentrically-mounted pistons and is well known in the art, as evidenced by the disclosure of U.S. Pat. No. 1,837,714 to Jaworowski dated Dec. 22, 1931 (see page 1, lines 29-67 and especially lines 62-67 and FIG. 8). The gap causes a loss of the pressure drop between the inlet and outlet conduits of the pump, thereby diminishing the pumping efficiency of the pump.
To address this gapping problem, some workers have attempted to modify the shape of the pistons. For example, U.S. Pat. No. 3,726,617 to Daido dated April 10, 1973 discloses a double rotary piston pump in which the rotors or pistons have a varying radius in order to accommodate the opposing gap-causing motions of the pistons and to insure that the pistons remain in contact at all positions in the pumping cycle. Likewise, U.S. Pat. No. 1,771,863 to Schmidt dated July 29, 1930 teaches the use of pistons having a cross-sectional profile which consists of arcs of circles of differing sizes (see page 1, lines 65-75).
Another approach has been to recognize that the gap may be accommodated by the use of slightly oversized pistons to allow for mutual contact even at the position where gapping is expected to be the greatest. However, when such oversized pistons rotate through each cycle or period of rotation, a high degree of compression is attained between the pistons in positions where gapping diminishes. To accommodate this compression, it is known to cover the contact peripheral surfaces of the pistons with a compressible material which, while maintaining contact without stress in the position where gapping would be the greatest, can compress to a sufficient degree to accommodate the relative closeness of the pistons in the aligned position. Such a solution is taught, for example, by U.S. Pat. No. 3,078,807 to Thompson dated Feb. 26, 1963. Still another solution taught in the art is to rotate the pistons at various speeds through the use of elliptical drive gears, as taught by Jaworowski, U.S. Pat. No. 1,837,714 dated Dec. 22, 1931. Other attempts at improving the performance of double piston rotary pumps include the modification of the piston surface to provide a ramp or step on the surface of the pistons as shown in U.S. Pat. No. 2,453,284, to Tornborg dated Nov. 19, 1948, col. 1, line 46 to col. 2, line 24, and U.S. Pat. No. 20,796 to Holly dated July 6, 1858, col. 2, lines 71-98).